Hi Cecil,
In the past, I've found dead roots do not show signs of RKN (i.e. when the roots die, the gals disappear.) Usually, this takes less than 2 hours for the gals to be gone once the root is separated from the tree. If you have had evidence of RKN in the hole before, they are probably still there.
The best way to check for RKN is on live roots.
~james